Abstract

Submerged leaf litter is a vital resource for many aquatic species in wood- land streams. However, forestry tends to lower the diversity of litter entering streams, which potentially impacts leaf-dependent species and the entire detritus-based food web. To overcome this problem, a guideline for sustainable forestry and ecosystem conservation lies in increasing litter diversity in managed forests. However, the way in and the extent to which such an increase changes the leaf decomposer communities are mostly unknown. In the present study, fungal communities were surveyed bimonthly in ten woodland headwater streams with contrasting tree diversity resulting in various proportions of leaf species in litter. A total of 79 aquatic hyphomycete species identified from conidia in stream water were listed. The differences in conidial abundance, richness and diversity between communities from the ten streams suggested a strong effect of local abundance and composition of leaf litter. Conidial abundance clearly distinguished two sets of streams, one dominated by oaks and hazel and the other by beech. The fungal richness in the ten streams showed a positive and significant correlation with the leaf species richness, accounting for 44 % of the total variance among streams. About 50 % more fungal species were found in streams with the highest leaf litter diversity, and several species were more frequent in such streams. No distinct species assemblages were however associated with any particular leaf species. These findings were interpreted as resulting from substrate preferences, but not specificity by the fungal species. This is supported by a manipulation in which the addition of a bulk of hazel leaves in a stream dominated by beech did not result in an increase in fungal richness over three months. Seasonal changes appeared to be the main factor controlling fungal species abundance. Overall, the results suggest a hierarchical pattern between riparian and aquatic biodiversity and provide an argument for the conservation of diverse tree species along streams.